Add flag for symbolic color names

This is a rough implementation for issue https://github.com/yihui/tikzDevice/issues/69. I'm waiting for feedback
As above -- reallocation should expand the array by a *constant factor* if necessary. I've found https://github.com/attractivechaos/klib, it's under MIT/X11 license. Or we switch to C++ and use the `std` library. Perhaps the easiest route would be to "misuse" some of R's internal data structures.

As a workaround, we could limit the size of the color map, perhaps through an option. After all, how useful is a huge color map?